The present invention relates to sports equipment, and more particularly to damping, controlling vibrations and affecting stiffness of sports equipment, such as a racquet, ski, or the like. In general, a great many sports employ implements which are subject to either isolated extremely strong impacts, or to large but dynamically varying forces exerted over longer intervals of time or over a large portion of their body. Thus, for example, implements such as baseball bats, playing racquets, sticks and mallets are each subject very high intensity impact applied to a fixed or variable point of their playing surface and propagating along an elongated handle that is held by the player. With such implements, the speed, performance or handling of the striking implement itself may be affected by the impact, and the resultant vibration may strongly jar the person holding it. Other sporting equipment, such as sleds, bicycles or skis, may be subjected to extreme impact as well as to diffuse stresses applied over a protracted area and a continuous period of time, and may evolve complex mechanical responses thereto. These responses may excite vibrations or may alter the shape of runners, frame, or chassis structures, or other air- or ground-contacting surfaces. In this case, the vibrations or deformations have a direct impact both on the degree of control which the driver or skier may exert over his path of movement, and on the net speed or efficiency of motion achievable therewith.
Taking by way of example the instance of downhill or slalom skis, basic mechanical considerations have long dictated that this equipment be formed of flexible yet highly stiff material having a slight curvature in the longitudinal and preferably also in the traverse directions. Such long, stiff plate-like members are inherently subject to a high degree of ringing and structural vibration, whether they be constructed of metal, wood, fibers, epoxy or some composite or combination thereof. In general, the location of the skier's weight centrally over the middle of the ski provides a generally fixed region of contact with the ground so that very slight changes in the skier's posture and weight-bearing attitude are effective to bring the various edges and running surfaces of the ski into optimal skiing positions with respect to the underlying terrain. This allows control of steering and travel speed, provided that the underlying snow or ice has sufficient amount of yield and the travel velocity remains sufficiently low. However, the extent of flutter and vibration arising at higher speeds and on irregular, bumpy, icy surfaces can seriously degrade performance. In particular, mechanical vibration leads to an increase in the apparent frictional forces or net drag exerted against the ski by the underlying surface, or may lead to a loss of control when blade-like edges are displaced so much that they fail to contact the ground. This problem particularly arises with modern skis, and analogous problems arise with tennis racquets and the like made with metals and synthetic materials that may exhibit much higher stiffness and elasticity than wood.
One practical approach for controlling vibration from arising has been to incorporate in a sports article such as a ski, an inelastic material which adds damping to the overall structure. Because of the trade-offs in weight, strength, stiffness and flexibility that are inherent in the approach of adding inelastic elements onto a ski, it is highly desirable to develop other, and improved, methods and structures for vibration control. Applicants have previously described in U.S. patent application Ser. No. 08/188,145 and corresponding published International Application WO95/20827 a modular packaged strain transducer unit which can not only change its own shape, but which couples strain across a surface. Applicants have furthermore described, in U.S. patent application Ser. No. 08/536,067 and corresponding International Application PCT/US96/15557 a construction wherein such strain transducer units are coupled in defined regions of a sports implement together with an active or passive circuit to damp, shift or otherwise control behavior of the implement under conditions of dynamic stimulation.
In implementing that technology, applicant created a sports damper wherein all or a portion of the body of a piece of sporting equipment has mounted thereto an electroactive assembly which couples strain across a region of the body of the sporting implement and alters the damping or stiffness of the body in response to strain occurring in the implement. Electromechanical actuation of the assembly adds or dissipates energy, effectively damping vibration as it arises, or alters the stiffness, changing the dynamic response of the equipment. The sporting implement is characterized as having a body with a root and one or more principal structural modes having nodes and regions of strain. The electroactive assembly is generally positioned near the root, to enhance or maximize its mechanical actuation efficiency. The assembly may be a passive component, converting strain energy to electrical energy and shunting the electrical energy, thus dissipating energy in the body of the sports implement. Alternatively it may be an active embodiment, in which the system includes an electroactive assembly with piezoelectric sheet material and a separate power source such as a replaceable battery. The battery is connected to a driver to selectively vary the mechanics of the assembly. For example, a sensing member in proximity to the piezoelectric sheet material may respond to dynamic conditions of strain occurring in the sports implement and provide output signals which are amplified by the power source for actuation of the first piezo sheets. A controller may include logic or circuitry to apply two or more different control rules for actuation of the sheet in response to the sensed signals, effecting different actuations of the first piezo sheet.
Applicant has constructed such a damper in a ski in which the electroactive assembly is surface bonded to or embedded within the body of the ski at a position a short distance ahead of the effective root location, i.e., ahead of the boot mounting. In a passive construction, the charge across the piezo elements in the assembly is shunted to dissipate the energy of strain coupled into the assembly, while in an active embodiment, a longitudinally displaced but effectively collocated sensor detects strain in the ski, and creates an output signal which is used as input or control signal to actuate the first piezo sheet. A single 9-volt battery powers an amplifier for the output signal, and this arrangement applies sufficient power for up to a day or more to operate the electroactive assembly as an active damping or stiffening control mechanism, shifting or dampening resonances of the ski and enhancing the degree of ground contact and the magnitude of attainable speeds. The foregoing technique is of general applicability; in other sports implements the piezoelectric element may attach to the handle or head of a racquet or striking implement to enhance handling characteristics, feel and performance.
As described in the aforesaid '067 patent application, using this resistive shunt control technique, the strain transducers are only able to effect a small level of damping, but this is applied over a broad frequency band. Thus, they are configured to continuously dissipate or redirect energy to prevent resonant excitation build-up, and the strain elements are preferably mounted in locations where they can capture strain energy from several excited modes. Further details of that construction are given in the aforesaid U.S. and International patent applications, all of which are hereby incorporated by reference.
However, in practice, an implement such as a ski is subject to very large disturbances at various frequencies depending upon the user and the environment. Thus, the shear-mounted strain element might not be able to affect the vibration levels occurring under some conditions, while in others practical experience and close observation may reveal particular states that could be advantageously controlled by coupled strain elements.
It is therefore desirable to increase the effectiveness of a strain element damper in a sports implement such as a ski.
It is also desirable to provide a dynamic strain element controller that is effective in the face of variations in the dynamics of the implement.
It is also desirable to provide a dynamic strain element controller that is effective in the face of variations occurring in electrical components used in the construction of the controller.
It is also desirable in particular to provide a strain element coupled to a ski and having an electrical control circuit tuned to a narrow ski frequency response band, wherein the response band encompasses a range of frequencies which may vary, due for example to velocity, terrain or device size and fabrication tolerances.
It is also desirable to provide a controller which enhances the levels of damping at one or more specific narrow frequency bands.